A wideband mobile communication system using complex modulation techniques, such as wideband code division access (WCDMA) and orthogonal frequency division multiplexing (OFDM), has large peak-to-average power ratio (PAPR) specifications and hence requires highly linear power amplifiers for its RF transmissions. Conventional digital predistortion (DPD) techniques have an operational bandwidth limitation.
Conventional DSP-based DPD schemes utilize FPGAs, DSPs or microprocessors to compute, calculate and correct the PA's nonlinearities: they perform fast tracking and adjustments of signals in the PA system. However, conventional DSP-based DPD schemes are challenged by variations of the linearity performance of the power amplifier over wide bandwidths due to the environment changing such as temperature and the asymmetric distortions of the output signal of the PA resulting from memory effects. Conventional DPD algorithms are based on a wideband feedback signal, they require a high speed analog-to-digital converter (ADC) in order to capture the necessary information. Multi-frequency band, or simply multi-band, applications can have their operating frequencies spaced significantly apart. Conventional DPD architectures use an ADC sampling rate that is greater than twice the nonlinear distortion bandwidth of the input signal. This sampling rate is typically more than double a factor of five times the operating bandwidth of the complex modulated signal. The factor of five accounts for the spectral regrowth attributed to the nonlinear distortion created by the power amplifier. This restriction on sampling rate, limits the feasibility of the conventional predistortion architectures to single band applications. Higher sampling rate ADCs have lower resolution, consume more power and are more expensive.